Cement

Cement is a pulverized material that develops binding forces due to a reaction with water

ASTM C 150, Standard Specifications for Portland Cement

Ordinary Portland Cement (OPC)

Portland cement is the name given to cement obtained by intimately mixing together calcareous (limestone and chalk) and argillaceous(Silica, Alumina and iron oxide) or other silica-, alumina and iron oxide bearing materials. Burning them at a clinkering temperature and grinding the resulting clinker

Blended Portland Cements

According to ASTM C 595; blended cement is defined as a mixture of portland cement and blast furnace slag (BFS) or a mixture of portland cement and pozzolanic material (most commonly fly ash, silica fume)

Modified Portland cement (Expansive

Cement)

Expansive cement contains hydraulic calcium silicates (CaO) that increase in volume significantly during the early hydrating period

These types of cement are specially used to minimize the cracks in concrete due to dry shrinkage

ü Hydraulic Cement(Product of hydration is stable under water, all types of OPC
)

ü Non-hydraulic Cement(Hydration product is not stable under water e.g. limestone)

All types of Portland cement are hydraulic cement

Types of Portland Cement

Type I(OP): General purpose. For use when the special properties specified for any other types are not required.

Type II(MC): Especially when moderate sulfate resistance or moderate heat of hydration is desired.

Type III(RHP): For use when high early strength is desired. (limit the C₃A content of the cement to maximum 15%)

Type IV(LHP): For use when low heat of hydration is desired.

Type V(SRP): For use when high sulfate resistance is desired. (Maximum limit of 5% on C₃A)

Cement Chemistry

Short Hand Notations for different compounds in OPC

·      C (CaO, calcium oxide)

·      A (Al₂O₃, alumina)

·      S (SiO₂, silica)

·      Ś(SO₃, sulfate)

·      H (H₂0, water) 

Reactive Compounds and short-hand names

Chemical Name	Chemical Formula	Shorthand Name	Mass (%)
Tri-calcium silicate	3Cao.SiO2	C3S	50-70
Di-calcium silicate	2Cao.SiO2	C2S	15-30
Tri-calcium aluminate	3CaO.Al2O3	C3A	5-10
Tetra-calcium aluminoferrite	4CaO.Al2O3.Fe2O3	C4AF	5-15
Calcium sulfate dihydrate(Gypsum)	CaSO4.2H2O	CŜH2	≈5

Chemical reaction between cement and water is called hydration of cement whereas the quantity of heat (in joules) evolved per gram of cement upon complete hydration of cement at given temperature is called the heat of hydration;

3C₃S +6Hè C-S-H + 3CH (120 cal/g)

2C₂S +4H è C-S-H + CH (62cal/g)

C₃A + 3CSH₂ + 26Hè C₆AŜ₃H₃₂(300 cal/g)

C₃A + C₆AŜ₃H₃₂ +4H è 3C₄AŜH₁₂

Manufacturing of cement

1. Crushing and Proportioning

Predetermined proportions of calcareous (limestone and chalk) and argillaceous (Silica, Alumina and iron oxide) are intimately mixed in the first step after quarrying. Mountains of rock are fed through crushers capable of handling pieces as large as an oil drum. The first crushing reduces the rock to a maximum size of about 15 cm. The rock then goes to secondary crushers or hammer mills for reduction to about 7.5 cm or smaller.

2. Raw milling & Blending

The next step in the process is to grind the above particles to a size of 90 microns or less which is done in a raw mill, a closed circuit ball mill equipped with high efficiency separator. After achieving the 90 microns size the fine grinded material also known as raw meal is sent to the continuous blending silos (CFC) for homogenization & extracted by means of load cell hopper for the next step which is feeding to the kiln pre heaters.

3. Pyro processing

The raw material is heated to exceeding 1,450 °C (2,700 degrees F) in huge cylindrical steel rotary kilns lined with special firebrick. Kilns are frequently as much as 3.7 M in diameter, large enough to accommodate an automobile and longer in many instances than the height of a 40-story building. Kilns are mounted with the axis inclined slightly from the horizontal. The finely ground raw material or the slurry is fed into the higher end. At the lower end is a roaring blast of flame, produced by precisely controlled burning of powdered coal, oil or gas under forced draft.

4. Burning and cooling

As the material moves through the kiln, certain elements are driven off in the form of gases. The remaining elements unite to form a new substance with new physical and chemical characteristics. The new substance, called clinker, is formed in pieces about the size of marbles.

Clinker is discharged red-hot from the lower end of the kiln and generally is brought down to handling temperature in various types of coolers. The heated air from the coolers is returned to the kilns, a process that saves fuel and increases burning efficiency.

5. Cement milling, Storage & Packing

Portland cement, the basic ingredient of concrete, is a closely controlled chemical combination of calcium, silicon, aluminum, iron and small amounts of other ingredients to which gypsum is added in the final grinding process to regulate the setting time of the concrete. Lime and silica make up about 85% of the mass. Common among the materials used in its manufacture are limestone, shells, and chalk or marl combined with shale, clay, slate or blast furnace slag, silica sand, and iron ore. The above mixture

Properties of Portland Cement

1. Fineness
2. Soundness
3. Consistency
4. Setting time
5. False set and flash set
6. Compressive strength
7. Heat of hydration
8. Loss on ignition
9. Density
10. Bulk density
11. Sulfate expansion